As this kind of devices that measure the air flow rate, various structures and types have been practically employed. Along with the sophistication of the engines, higher accuracy for the measurement of the flow rate has been required. In particular, in order to reduce the sensor error in the engine rippling, the backward flow detection method of the flow sensor has been examined. A bobbin type flow sensor typified by the one disclosed in PTL 1 has conventionally been the mainstream but the characteristic of such a flow sensor changes in the positive direction regardless of whether the flow is the forward flow or the backward flow. Therefore, under the rippling condition where the backward flow is caused, the rich error (the error that the average flow rate becomes high) occurs in the bobbin type flow sensor (see FIG. 1).
In contrast to this, the thermal flow sensor that can detect the directions of the forward flow and the backward flow can output the backward flow waveform without being inverted; therefore, the error can be suppressed. Such a thermal flow sensor with a general structure is, as disclosed in PTL 2, the quick responsive flow sensor in which the resistor and the insulating film are provided for the silicon substrate (wafer) through the semiconductor process and the wafer is thinned through the back etching, so that the thermal capacity is suppressed.